The present invention relates to electrodes to be used for carrying out spot welding of metallic materials having a high electric conductivity and a high thermal conductivity, and more particularly to an electrode structure that is preferable for small current welding.
Spot welding is a welding method in which two metal sheets are superposed on each other, then an electric current is made to flow through these metal sheets while pinching and pressing them with a pair of circular-rod-shaped electrodes, and the metal sheets are joined by heating and melting them with Joule's heat generated by the electric current. In the case of carrying out welding of metal sheets made of aluminum, aluminum alloy, magnesium alloy or the like having a high electric conductivity and a high thermal conductivity through this spot welding process, since the generated amount of Joule's heat is small and moreover the generated heat is dissipated through the members to be welded having a good thermal conductivity, a heavy current and a large pressing force (a pressing force exerted by the electrodes) are necessitated, and furthermore, the welding must be completed within a short period of time. Accordingly, a large-sized transformer is necessary, also in order to realize a large pressing force, a large-sized spot gun which deteriorates a workability must be employed, and therefor, it was difficult in the prior art to portabilize a welding machine.
Furthermore, even if a heavy current is used, upon carrying out multi-spot spot welding there was a shortcoming that due to the good electric conductivity of the members to be welded, a shunt current passing through an already completed welding point is produced, and the welding at the presently welding location could not be carried out soundly.